Pure Charlie and speed (6)
I believe the answer is:
chaste
'pure' is the definition.
('chaste' can be similar in meaning to 'pure')
'charlie and speed' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' means one lot of letters go next to another.
'speed' becomes 'haste' (haste is a kind of speed).
'c'+'haste'='CHASTE'
(Other definitions for chaste that I've seen before include "Cheats (anag.)" , "Like Diana" , "Sexually virtuous" , "Undefiled" , "He-cats could be virtuous, strangely" .)